Heterojunction bipolar transistors (HBT) and Schottky diodes are often incorporated into a single semiconductor structure. For example, a Schottky diode may be included in a fast diode sampling bridge in HBT-based mixed-signal circuits. In another example, Schottky diodes may be used for level shifting in the logic core of the circuits.
The usual approach to integrating the HBT and the Schottky diode is to build the Schottky diode from the same epitaxial layers that are used as the collector of the HBT. While such a structure is readily fabricated, it requires the collector material to perform a dual function, which results in a compromise of one or the other function. The common collector materials used for high-speed HBTs make poor Schottky diode barrier layers due to low bandgaps (in InGaAs collectors) or to poor surface Fermi-level pinning and low barrier heights (in InP collectors). The low bandgap collectors have poor breakdown and low turn-on voltages. The poor surface pinning leads to process-dependent barrier heights and to poor uniformity and reproducibility.
The inventor has determined that if the collector material of the HBT is modified to improve the performance of the Schottky diode in such a construction, the transport and breakdown properties of the HBT collector are degraded. The result is that the conventional integrated HBT and Schottky diode has compromised properties that are not optimal for either the HBT or the Schottky diode.
There is a need for an improved approach to the structure of the integrated HBT and Schottky diode that permits the two devices to function together, but allows their separate optimization. The present invention fulfills this need, and further provides related advantages.